1. Technical Field
The present disclosure relates to electrosurgical devices and, more particularly, to electrosurgical tissue ablation systems capable of detecting excessive bending of a probe shaft and alerting a user.
2. Discussion of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, thermal, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator. In tissue ablation electrosurgery, the radio frequency energy may be delivered to targeted tissue by an antenna or probe.
There are several types of microwave antenna assemblies in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors, which are linearly aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include a helically-shaped conductor connected to a ground plane. Helical antenna assemblies can operate in a number of modes including normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis. The tuning of a helical antenna assembly may be determined, at least in part, by the physical characteristics of the helical antenna element, e.g., the helix diameter, the pitch or distance between coils of the helix, and the position of the helix in relation to the probe assembly to which it is mounted.
The typical microwave antenna has a long, thin inner conductor that extends along the longitudinal axis of the probe and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the axis of the probe. In another variation of the probe that provides for effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or combinations thereof.
Invasive procedures and devices have been developed in which a microwave antenna probe may be either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growth of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C., thermal damage to most types of normal cells is routinely observed. Accordingly, great care must be taken not to exceed these temperatures in healthy tissue.
Because of the small temperature difference between the temperature required for denaturing malignant cells and the temperature normally injurious to healthy cells, a known heating pattern and precise temperature control is needed to lead to more predictable temperature distribution to eradicate the tumor cells while minimizing the damage to surrounding normal tissue. Excessive temperatures can cause adverse tissue effects. During the course of heating, tissue in an overly-heated area may become desiccated and charred. As tissue temperature increases to 100° C., tissue will lose water content due to evaporation or by the diffusion of liquid water from treated cells, and the tissue becomes desiccated. This desiccation of the tissue changes the electrical and other material properties of the tissue, and may impede treatment. For example, as the tissue is desiccated, the electrical resistance of the tissue increases, making it increasingly more difficult to supply power to the tissue. Desiccated tissue may also adhere to the device, hindering delivery of power. At tissue temperatures in excess of 100° C., the solid contents of the tissue begin to char. Like desiccated tissue, charred tissue is relatively high in resistance to current and may impede treatment.
Microwave ablation probes may utilize fluid circulation to cool thermally-active components and dielectrically load the antenna radiating section. During operation of a microwave ablation device, if proper cooling is not maintained, e.g., flow of coolant fluid is interrupted or otherwise insufficient to cool device components sensitive to thermal failure, the ablation device may be susceptible to rapid failures due to the heat generated from the increased reflected power. In such cases, the time to failure is dependent on the power delivered to the antenna assembly and the duration and degree to which coolant flow is reduced or interrupted.
Cooling the ablation probe may enhance the overall heating pattern of the antenna, prevent damage to the antenna and prevent harm to the clinician or patient. During some procedures, the amount of cooling may not be sufficient to prevent excessive heating and resultant adverse tissue effects. Some systems for cooling an ablation device may allow the ablation device to be over-cooled, such as when the device is operating at low power settings. Over-cooling may prevent proper treatment or otherwise impede device tissue effect by removing thermal energy from the targeted ablation site.
Microwave ablation probes come in many lengths with probes exceeding 30 cm being considered. The probe shaft typically includes a glass-fiber cooling jacket which is the main structural member of the probe. There is a certain degree of flexibility inherent in the jacket. However, excessive bending loads on the shaft can cause a sudden failure to occur, resulting in the jacket snapping at the point at which maximum load is placed on the jacket.
In several designs of the shaft, a steel hypo-tube is fitted inside the jacket in the proximal end which functions as a stiffener. The hypo-tube presents design compromises to the cooling system and it is not generally desirable. However, if the hypo-tube were to be removed, bending loads on the shaft are likely to approach a point at which fracture of the cooling jacket is likely to occur. Even with the hypo-tube incorporated within the shaft or other stiffener, it is desirable to prevent excessive bending of the probe shaft during electrosurgical procedures.